1. Field of the Invention
The present invention relates to a luminescent material, a method of manufacturing a luminescent material, and a light-emitting device.
2. Description of the Related Art
Conventionally, a luminescent material of the YAG:Ce series that emits yellow light has been used for a white LED. As this luminescent material emits only weak light upon excitation by near-ultraviolet radiation, a light source is limited to blue-emitting one. In addition, because red component intensity is weak in an emission spectrum, for making a white light having a low color temperature, color rendering properties are considerably reduced. Although there have been made attempts to improve the color rendering properties by mixing red luminescent materials of a nitride series, homogeneous mixing is difficult since the chemical composition or crystal system of constituents of a yellow luminescent material and of a red luminescent material are different. Yet, yellow luminescent materials and red luminescent materials have to be synthesized with different equipment and by different methods, and the latter of the nitride series needs to be synthesized at high temperatures of 1600° C. or higher. Thus, the manufacturing cost is very high.
As a luminescent material that is excited by light having a wavelength ranging from 360 to 500 nm and that emits green light to yellow light, a divalent europium-activated alkaline earth orthosilicate luminescent material (Srx, Bay, Caz, Euw)2SiO4 has been known. Emission colors of the luminescent material vary, depending on the kind of alkaline earths or a proportion thereof. A white LED to which such the luminescent material is applied is also proposed.
Such luminescent material can be manufactured with standard equipment or by a standard method even though emission colors are different. The sintering temperature necessary for the manufacture are 1000 to 1500° C., and if the temperature is higher than this, the cost needed for the equipment would be higher. In addition, although emission colors are different, homogeneous mixing is easier because a crystalline structure is identical and chemical and physical properties are similar. Therefore, if emission of a peak wavelength in a red region is achieved, a luminescent material showing this emission and a yellow or green emitting luminescent material are mixed, thereby enabling improvement of color rendering properties even though the light is white of a low temperature. It is currently reported, however, that the peak wavelength of emission is 505 to 598 nm.